Analog to digital conversion is a commonly used technique wherein a continuous signal is converted to a digital signal for the purpose of signal processing. An analog to digital converter (ADC) is often used for such a conversion. ADCs typically have a limited number of bits available, and thus a limited conversion range, to perform analog to digital conversions. Automatic gain control (AGC) is therefore used to adjust the power level of an incoming signal such that the ADC will receive signals at a fixed level; thus, the number of bits required by the ADC to perform conversions may be dramatically reduced. The AGC controls the gain of a system in order to maintain an adequate performance over a range of input signal levels.
Gain will be discussed herein in terms of decibels (dB). A dB is typically used to describe the ratio between two measurements of electrical power, which may be arithmetically added and subtracted. A dBm represents an absolute unit of electrical power. A dBm may be defined as A=10*log 10(P2/(1mW)), where A is the absolute unit of power and P2 is a measurement of electrical power. The ratio of power may be defined as P2/(1mW)=10^(A/10). For example, 1 dBm is one dB greater than 0 dBm, or about 1.259 mW (1.259=101/10).
A canonical form of a conventional AGC scheme in a digital communications system 100, is illustrated in FIG. 1. The system 100 comprises a Variable Gain Amplifier (VGA) 103, that receives an input signal 101. The AGC 105 receives a digital signal 106, digitized via an ADC 107. The AGC 105 supplies information to the VGA 103 via a feedback connection 104. The information supplied by the AGC 105 is used in adjusting the gain supplied to the input signal 101. It should be appreciated that the gain adjustment affects the average total power of the signal and not the instantaneous power of the signal. Thus, the gain adjusted signal will still comprise its unique signal properties since its instantaneous power will be intact. A modem 109 is typically used to demodulate the signal in order to produce bits 113.
As discussed above, when designing a digital communication system, the dynamic range must be put into consideration. The dynamic range of the input signal may be extremely large; 802.11 modems typically support close to 90 dB of dynamic range. Area and power requirements for an ADC typically increases by four times every 6 dB. Hence, a large ADC dynamic range is extremely expensive.
A solution for this problem, as previously mentioned, is to reduce the dynamic range seen at the ADC by performing automatic gain control. An ideal AGC switches in the right amount of analog gain such that the signal power at its output A, FIG. 1, is always the same, regardless of the input signal level. Hence, an ideal AGC completely eliminates signal dynamic range. Thus, the AGC is essential in such a system as it controls the gain of an incoming signal in order to bring the signal to a suitable level for conversion or any other form of signal processing.
As an example, consider a system that must receive single channel signals from −100 dBm to −10 dBm, 90 dB of dynamic range. To accommodate this range, a VGA is used that must be set to 0 through 90 dB of gain. Therefore, for a signal which is (−10−X) dBm, X dB of gain is typically switched into the signal. Using this technique the output always stays at −10 dBm. Otherwise, assuming 1 bit is required to convert a 6 dB analog signal to a digital signal, a maximum of 15 bits would be needed to convert a −90 dBm signal. A conversion requiring 15 bits is technically very difficult. Thus, if a −40 dBm signal arrives in the system, 30 dB of gain is added to the signal in order to obtain the optimum value, dramatically reducing the amount of bits required for the conversion.
One way of building such an AGC is to simply cycle through all possible gain settings, for example in 2 dB steps, and stop when the desired signal level is reached. One might choose to use a binary search instead of a linear one to increase the speed of the acquisition.